Process of producing malleable iron



- dritic crystals of austenite, an

Patented July 6, 1926.

UNITED STATES PATENT OFFICE.

HARRY WILLIAMS ALFRED BOEGEEOLD, OF DAYTON, OHIO, ASSIGNOBS TO GENERAL MOTORS RESEARCH CORPORATION, 01 DAYTON, OHIO, AOOBPORATIOH or nnmwm.

Io Drawing.

The present invention relates to improve-.

ments in making malleable iron castings and has among, its objects, greatly chea ening j the process and at the same time pro ucing i an excellent commercial malleable material.

. Another object is to reduce to a small portion thereof, the time heretofore considered necessary to produce such castin s.-

' The structural composition malleable 1 iron produced by the regular long anneal is a ferrite matrix'containing nodules of temper carbon. 7 The size of the ferrite grain,

the number and size of the temper carbon- .stit'uents in the form just described is a result of the composition and heat treatmentof the iron. The first raiuirement of a casting -intended for mallea leizing is that it shall be of such composition that no free carbon p will be precipitated in flake form during Cthe solidification and subsequent cooling after casting. .In other words, the casting must be white iron. The microstructure of such an iron is composed of (primary dena secondary structure of austenite and massive eementite (iron carbide or Fe,C) in eutectic form.

- 1 Upon annealing such a structure, the carbon a is precipitated in the nodular form required for malleable iron. In the annealing of ordinary malleable iron,-the first stage at 1700 F. changes the secondary structure in eutectic form of massive cementite and aus- 'tenite into temper carbon in a matrix of austenite. After the massive cementite of the secondary eutectic structurehas completely disa' peared, further anneal-ingat 1700 F.

pro uces. no change since approximately 1.3% carbon is soluble in iron at this temperature. The annealing temperaturejsgthen dropped very slowly to just below the lower critical temperature during which time the carbon in execs of eutectoid composition is progressively thrown out of solution in the ,quired injthe' origin'a process .descri raocnss or raonucme uALLmLn men.

' Application filed Kay 21, 1924. Serial No. 714,882.

form of cementite, which is then decomposed into graphite and ferrite. The iron is then held at just below the lower critical temperature (approximately 1340 F.) long enough causing the cementite tobreak down into iron and carbon. too low, the time of annealing will be increase mercially practicable. If the silicon is raised too high (above 1.3%) in the heretofore usual process in order to accelerate the annealing operation, there is dan er of carbon precipltating in the form of ake graphite during the solidification of the casting. Even though this occurs to a very small extent, these flakes of graphite act-as loci of carbon precipitation in the 'subse uent anneal, increaslng the size of these akes and consequently destroying the malleability of the casting. Therefore, commercially sand cast malleable iron must be held to a range If the silicon content isof silicon content which requires long annealing periods to produce temper carbon.

Heretofore in producin malleable castings it has been the usual, If not the univering which time the articles are'brought to a 'tempprature of 1700 F.(second), a period of a out-4O hours-y-during which the temperature of 1700 F. must be maintained,-

too much to make annealing com- .heating up period of about 30 hours-dur- (third) slowly coolin to the lower critical temperature and hol mg' this for about 85 hours, and (fourth) the period of cooling to a temperature low enough to permit handling, about five hours. 1 The present process, on the-other hand, reduces the actual annealing timeexclusive of heating and coolin from 7 5 hon to under 15 hours. Goodresults obtained in four hours and can in some "caaesbe reduced to two hiiura.

Contrary to the usual practlco, 1t has been found that, if the carbon precipitating agent, such as silicon, is increased to from 2 to l the carbon maintained within the range of 1.7 to 3.5%, and the casting operation carried out in such a fashion as to obtain a white iron, the casting may then be converted-to the malleable condition by annealing only a short time at a temperature of 1500 to 1700 F.

It has also been found that the content of the carbon precipitating agent, for example, silicon, bears a, direct relation to the time required for anhealing, the other conditions being maintained.

In carrying out the process which is based upon the foregoing discoveries, use is made of a mold in which the cooling of thecasting,

.ina. be controlled so as to accomplish this coo ing very rapidly and thus obtain a product which is entirely White iron or one which is complete chilled. solidification and cooling of t e iron in the casting operation,

through the temperature range where carbon -is precipitated in flake form, or as flake graphite, should be as rapid as possible. After the lower limit of this temperature range has been reached, the casting may be taken immediately to the annealing furnace or it may be allowed to cool to room temperature and subsequently annealed.

The preferred iron composition for reducing the annealing time to periods of 15 hours or less, will contain from 2 to 4 silicon, 1.7 to 3.5% carbon, and a manganese, sulphur, and phosphorus content-as low as practicable. The sulphur will be preferably not over .10 and phosphorus preferably not over 20%. The manganese content is preferably not over .7 0% but amounts up to 1.00% do not seem-to interfere greatly with the process.

- If the silicon content is as low as 2%, the annealing time will be increased considerably but still be well-below the time found necessary-in the conventional or heretofore usual process.

A preferred iron composition is one that .will require an annealing period of around four. hours and such a composition will have'the silicon within the range of 2.5% to 3.25%, the carbon between 2.25% and 2.5% with the hereinbefore mentioned limits on manganese, phosphorus and sulphur.

The following specific compositions have been used and, when annealed for the time indicated, have given excellent quality malleable iron.

' Emmnple N0. 1.

Carbon 1.76%, silicon 3,97%, sulphur 024%, phosphorus 004%, manganese 23%, iron the remainder. Annealing time -two hours.

' Example No. 2. Y

Carbon 2.66%, silicon 2.77%, sulphur .031-%, phosphorus 006%, manganese .18%,

.noted that any precipitation of ear iron the remainder. Annealing timefour hours.

As stated above the quantity of the carbon precipitating agent in the iron bears a direct relation to the length of the annealing timeas this quantity decreases the annealing time increases.

The quantity of carbon precipitating agent also bears a direct relation to the rate of cooling necessary to prevent graphite precipitation in flake form-as this quantity increases. the rate of cooling necessary to pro duce white iron. also increases. that is. the period of cooling becomes briefer.

From these two relations it is obvious that the best proportion of the precipitating agent will depend to some extent at least upon the efficiency of the mold in cooling the casting. If the mold is so designed that a given rate of cooling cannot be exceeded the proporition of the precipitating agent should be determined accordingly and 1t is from such relations that the preferred quantity mentioned hereinbefore has been determined. although it is obvious that a change in cooling rate occasioned by mold design will fix other proportions as the best in a cing before annealing. The extra silicon is present in solution in the iron and perhaps combined with the cementite and does not change the appearanceof the constituents. When the casting is subs uently annealed, the high percentage of si icon causes the cementite to break down very rapidly into temper carbon and ferrite.

Among the carbon precipitating agents which may be used in the present process are silicon, aluminum, nickel, and co r, or combinations of these elements. Si men may be considered as the preferred one and has been given in the s ific examples.

Further, although it 12;: been indicated above that, when castin the ironto be malleableized, conditions s ould be controlled so as to produce a white iron, it shoulg be uring the casting operation will not interfere with the annealing unless it preci itates in theflake graphiteform. Precipitation of nodular carbon at this period is not objectionable.

Finally, it should be noted that the invention is not to be limited to the precise proportions and procedure mentioned as these may undoubtedl be varied within the sec of the claims w ich follow.

lgliat is claimed is as follows 1. The process of making malleable iron castings which consists in pouring into a mold iron containing a carbon precipitating constituent; cooling the casting at such rapid rate as to prevent precipitation of flake graphite; and then annealing the casting; the proportion of carbon precipitating constituent being substantially in excess of that which can be included in malleableizable iron poured in sand. molds and cooled at the usual rate.

2. The process of making malleable iron castings which consists in pouring into a chill mold, iron containing a proportion of carbon precipitating constituent substantially in excess of the amount that can be included in malleableizable iron poured into sand molds, and then annealing the castin 3. 1518 process of making malleable casttin'gs which consists in making-a white iron casting from an iron mixture containing more than 1.3% silicon, then annealing the casting.

4. The process of making malleable iron castings which consists in preparing a molten mixture of iron containing more than 1.3% of silicon; pouring the molten mixture into a mold of sufiiciently high heat conductivity to chill the castin so rapidl that substantially no flake car 11 will A precipitatcd, then annealing the castings which consists in 5. The process of maln'ng malleable iron chill mold, molten ironconta m 2.00 to 4.50% silicon, then annealing t ecasting. 6. The process of making malleable iron castings which consists in pouring into a pupinurinfgmto a metallic mold, molten iron containing more than 1.70% of carbon and more than 1.3% of silicon, then annealing the casting.

7. The process of making malleable iron castings which consists in pouring iron containing from 2% to 4.50% silicon and 1.70% to 3.50% carbon into a metallic mold and subsequently annealing the casting.

8. The rocess of making malleable iron castings w ich consists in preparing a molten mixture of iron containing upward of 1.70% of carbon and upwards of 2%"of silicon, pouring the molten mixture into a mold of sufliciently high heat conductivity to chill the casting so rapidly that substantially no flake carbon will be precipitated, then annealing the casting at a temperature of about 1500 to 1700 F. for a period not exceeding fifteen hours. I

9. The proces of producin'g malleable iron articles which consists in casting the article of iron containing from 2.00 to 4.50% Si, 1.70 to 3.50% C, and as low as practicable in Mn, 15' and S content under conditions to give a white iron and subsequently annealing for a short period of time.

10. The process of producing malleable iron articles which consists in casting the article in a metallic mold and subsequent- 1y annealing for a short period of time.

11. The process of producing malleable iron articles which consists in casting the article of iron containing from 2.00 to 4.50%-

Si, 1.70 to 3.50% C, and as low as practicable in Mn, P, and S content in a metallic.

mold and subsequently annealing for a short period of time.

In testimony whereof we hereto afiix our signatures.

HAR'RY M. WILLIAMS. ALFRED nnononnom). 

